1. Field of the Invention
The field of the invention is reduction of NO.sub.x and particulate matter (PM) emissions from Diesel-cycle engines. The field of application is primarily in internal combustion engines for motor vehicles.
2. Prior Art
The growing use of Diesel-cycle engines in motor vehicles greatly adds to the atmospheric presence of pollutants such as oxides of nitrogen and particulate matter. Conventional Diesel-cycle engines emit nitrogen oxide (NO.sub.x) and particulate matter (PM) substantially in excess of the emissions from Otto-cycle (e.g., gasoline) engines, yet Diesel-cycle engines achieve substantially better fuel economy. Because of the higher fuel economy, Diesel-cycle engines dominate the heavy-duty truck market and much of the off-road commercial vehicle market, with growing penetration into light duty trucks. Thus, technology which could substantially reduce NO.sub.x and PM emissions from Diesel-cycle engines is highly desired.
Two key features of Diesel-cycle engines are the absence of substantial throttling of the intake charge (i.e., air or a mixture of air and recirculated exhaust gas) and the direct injection of fuel into the combustion chamber. A third important feature of most modern Diesel-cycle engines is a turbocharger, usually followed by a charge cooler, to supply pressurized intake charge to allow increased specific power output. The turbocharger usually includes a turbine compressor driven by an exhaust gas turbine expander. During a command for a rapid rise in engine torque, increased fuel can be supplied almost instantaneously. However, if the engine is currently operating with high exhaust gas recirculation (EGR), there is a reduced quantity of oxygen available which will not allow maximum fuel injection without poor combustion and increased PM emissions. Also, until the exhaust energy level is increased to the level associated with the higher torque output, the turbocharger is unable to supply the increased boost pressure (and hence more mass of oxygen) that will ultimately be available at the new equilibrium (commonly called "turbo-lag"), and again a constraint must be placed on the maximum fuel injection quantity until the system responds with an increased mass of oxygen.
With conventional technology, it is especially difficult to quickly adjust the quantity of exhaust gas entering the combustion chamber with the charge air, because: (1) the response time of the EGR flow control valve is relatively long compared to the combustion cycles of the engine, and (2) the time required to "purge" the previously desired exhaust gas and air mixture from the intake system through the engine is also relatively long and may take several combustion cycles before the newly desired mixture can be established.